Congratulations on your new Iron Tanks weightlifting belt!
It might feel quite stiff and rigid. Particularly if it's a 13mm model belt, these are very tough.
The best way to break in and get used to your belt is simply by wearing it. The sweat and perspiration after a few workouts will eventually soften the belt so it becomes more comfortable and contoured. This process might take 4-5 weeks for some, and if it's your first belt - it is not unreasonable to feel quite uncomfortable at first.
When you get your belt, it is advised to put some slight curvature in it to aid the break in process. The best method of doing this is by standing upright, putting your leg at 90 degrees on a chair in front of you and curving the belt over your thigh up and down along the belt. This will help the leather fibres break down a little initially. Please see the above image for further clarification.
Do not fold your belt in half aggressively (pictured below) / put it in the dishwasher etc as frequently mentioned on the internet as a break in method. The break in period is a gradual one.
All the best in your lifting adventures.
Team Iron Tanks